Nth Highest Salary

Write a SQL query to get the nth highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

For example, given the above Employee table, the nth highest salary where n = 2 is 200.
If there is no nth highest salary, then the query should return null.

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
  declare M int;
  set M=N-1;
  RETURN (
      # Write your MySQL query statement below.
      #select distinct salary from Employee order by salary desc limit n,1
      SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT M, 1
  );
END

和Second Highest
Salary思路一样，但是在这里考察函数的编写。

注意：

函数里，不能定义n这样的变量，其次在select语句里不能使用limit N-1,1这样的语句